Intense theoretical and experimental interest has been focused on the possibility of new low-compressibility materials with bulk moduli and hardness exceeding that of diamond. Carbon nitrides have been proposed as superhard materials on the basis of empirical systematics [1]. First-principles calculations have suggested that a hypothetical material, .beta.-C.sub.3 N.sub.4 may have a bulk modulus somewhat lower than that of diamond [2,3]. These results have motivated theoretical calculations [4-8] and experimental efforts to synthesize and characterize this compound [10-20]. Amorphous C--N films have been synthesized [14,16,20] and small crystallites have been found in some of these films [15,17-19]. Electron diffraction patterns of these crystallites were indexed as the .beta.-C.sub.3 N.sub.4 structure. It has also been suggested that a better fit is to phases of graphite [9]. Other forms of carbon nitride with high hardness have been suggested including a fullerene-like carbon nitride [21] and a crystalline carbon nitride composite [22].